Great question! To put it simply, Olympus Mons was formed via. basaltic shield volcanism. More on this topic can be found here: 

https://en.wikipedia.org/wiki/Shield_volcano. 

Also, from Science Daily: "Shield volcanoes are formed by lava flows of low viscosity - lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano."

The special case for Olypus Mons is that it is simply the largest shield volcano in the solar system, and one may ask why? This is most likely due to hot spot location originating from the core mantle boundary (CMB) of Mars, while there was still a convective layer generating a magnetic field to initiate the instabilities in the CMB. The fixed martian crust allowed this hotspot to erupt in one location. This formation likley occured early in Mars history; Noachian to early Hesperian, when there was an abundant liquid ocean in the northern hemisphere, and Olympus Mons would have been a large Volcanic Island, exemplifying the big island of Hawaii. I've attached an 800 pixel colored DEM of Olympus Mons showing its relationship on the Tharsis Bulge, on the right in red.

There are also models that show a more recent volcanic source for the magma of Olympus Mons potentially derived from internal heating and small crustal shortening (tectonic compression) events.

Here's more research for you too:

https://www.researchgate.net/publication/223609730_Shape_rheology_and_emplacement_times_of_small_martian_shield_volcanoes

https://www.researchgate.net/publication/228791886_Secondary_Hawaiian_volcanism_formed_by_flexural_arch_decompression

https://www.researchgate.net/publication/236632554_Strengths_and_strain_energies_of_volcanic_edifices_Implications_for_eruptions_collapse_calderas_and_landslides

I've also attached an example maps of ancient Mars and terraformed Mars, as well as an ocean volume estimate publication. Enjoy!

Article Shape, rheology and emplacement times of small martian shiel...

Article Secondary Hawaiian volcanism formed by flexural arch decompression

Article Strengths and strain energies of volcanic edifices: Implicat...